Revealed: the violent past of Andromeda, our neighbouring galaxy

3 October 2019
The cannibalistic giant galaxy is headed our way
A team of astronomers, led by Professor Geraint Lewis, has revealed two periods over billions of years where Andromeda shredded incoming galaxies to create star clusters. Our galaxy is next on the menu.

Andromeda's violent past uncovered

Video: Sebastian Zentilomo

Astronomers have pieced together the cannibalistic past of the neighbouring large galaxy Andromeda, which has set its sights on our Milky Way as the main course.  

The galactic detective work found that Andromeda has eaten several smaller galaxies, likely within the last few billion years, with left-overs found in large streams of stars.

ANU researcher Dr Dougal Mackey, who co-led the study with Professor Geraint Lewis from the University of Sydney, said the international research team also found very faint traces of more small galaxies that Andromeda gobbled up even earlier, perhaps as far back as during its first phases of formation about 10 billion years ago.

“The Milky Way is on a collision course with Andromeda in about four billion years, so knowing what kind of a monster our galaxy is up against is useful in finding out its ultimate fate,” said Dr Mackey from the ANU Research School of Astronomy and Astrophysics.

“Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalised many more galaxies, possibly larger ones.”

Signs of ancient feasting are written in the stars orbiting Andromeda, with the team studying dense groups of stars, known as globular clusters, to reveal the ancient mealtimes. 

“By tracing the faint remains of these smaller galaxies with embedded star clusters, we’ve been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times,” Dr Mackey said. 

Professor Geraint Lewis in his University of Sydney office. Photo: Louise Cooper

Professor Geraint Lewis in his University of Sydney office. Photo: Louise Cooper

Something very weird  

The discovery presents several new mysteries, with the two bouts of galactic feeding coming from completely different directions.

“This is very weird and suggests that the extragalactic meals are fed from what’s known as the ‘cosmic web’ of matter that threads the universe,” said Professor Lewis from the Sydney Institute for Astronomy and University of Sydney School of Physics.

“More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre ‘plane of satellites’, an unexpected alignment of dwarf galaxies orbiting Andromeda.”

Dr Mackey and Professor Lewis were part of a team that previously discovered that such planes were fragile and rapidly destroyed by Andromeda’s gravity within a few billion years.

“This deepens the mystery as the plane must be young, but it appears to be aligned with ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation,” Professor Lewis said.

“We’re going to have to think quite hard to unravel what this is telling us,” he said.

Dr Mackey said studying Andromeda also informed our understanding of the way our galaxy has grown and evolved over many billions of years. 

“One of our main motivations in studying astronomy is to understand our place in the universe. A way of learning about our galaxy is to study others that are similar to it and try to understand how these systems formed and evolved,” Dr Mackey said.

“Sometimes this can actually be easier than looking at the Milky Way, because we live inside it and that can make certain types of observations quite difficult.”

The study, published in Nature, analysed data from the Pan-Andromeda Archaeological Survey, known as PAndAS.

“We are cosmic archaeologists, except we are digging through the fossils of long-dead galaxies rather than human history,” said Professor Lewis, a leading member of the survey.

The team involved institutions from Australia, New Zealand, the United Kingdom, Netherlands, Canada, France and Germany utilising the Canada-France Hawaii Telescope, the Gemini North Telescope, the Keck Telescope, the NOAO Mayall 4m telescope and the William Herschel Telescope.

Andromeda's collision with the Milky Way

NASA video


Professor Geraint Lewis acknowledges the support of a Partnership Collaboration Award between the University of Sydney and the University of Edinburgh and Dr Dougal Mackey acknowledges support of the Australian Research Council Future Fellowship program.

The research benefited from funding from the French National Research Agency and the Marsden Fund of the Royal Society of New Zealand. Dr Mackey and Professor Lewis appreciate the hospitality of the Royal Observatory, Edinburgh.

Some data in the research were obtained at the W. M. Keck Observatory, Canada France Hawai’i Telescope and the Gemini Observatory in Hawai’i. The authors recognise and acknowledge the significant cultural role and reverence that the summit of Maunakea has always had with the indigenous peoples of Hawai’i.

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